Review Question - QID 217194

This patient with a history of head trauma, epidural hematoma (biconvex lesion that does not cross suture lines), and a medialized left uncus most likely has a left uncal herniation. This may lead to compression of the left oculomotor nerve and disruption of parasympathetic inflow to that eye, resulting in mydriasis of the left eye.

The intracranial space has a fixed volume; therefore, increases in brain volume from a tumor, localized hemorrhage, or changes in CSF volume may lead to a pressure differential across fixed intracranial structures. When native compensatory autoregulation is overwhelmed, herniation may result. In a common type of brain herniation, the uncus, the innermost portion of the temporal lobe, can herniate across the tentorial notch. This may lead to compression of the ipsilateral posterior cerebral artery (PCA), corticospinal tract, and/or oculomotor nerve. Clinically, these may manifest as contralateral homonymous hemianopia, contralateral weakness, and ipsilateral oculomotor nerve palsy respectively. In addition, herniation of the ipsilateral uncus can compress the midbrain, indirectly leading to compression of the contralateral corticospinal tract. This results in the “false localizing sign” of ipsilateral weakness. The peripheral fibers of the oculomotor nerve provide motor innervation, while the central fibers provide parasympathetic outflow. Therefore, compression of cranial nerve III usually leads to palsy of muscles supplied by the oculomotor nerve (medial rectus, superior rectus, inferior rectus, inferior oblique, and levator palpebrae); in some severe cases, loss of parasympathetic inflow may also lead to mydriasis.

Gilardi et al. review the clinical presentation and radiologic findings of cerebral herniation, including extracranial herniation, subfalcine herniation, transtentorial herniation, and tonsillar herniation.

Incorrect Answers:
Answer 1: Hydrocephalus of the right lateral ventricle may be seen in subfalcine herniation. Subfalcine herniation can lead to compression of the contralateral interventricular foramen (foramen of Monro), which in turn can lead to isolated hydrocephalus of the contralateral lateral ventricle.

Answer 2: Right leg weakness may be seen in subfalcine herniation. Herniation of the cingulate gyrus of the frontal lobe under the falx cerebri may lead to compression of the ipsilateral anterior cerebral artery. In turn, this leads to contralateral weakness of the right lower extremity only.

Answer 3: Miosis of the left eye may be seen as part of central herniation, in which the diencephalon and parts of bilateral temporal lobes herniate undergo downward transtentorial herniation. Compression of the periaqueductal structures leads to bilateral miosis with paralysis of upward eye movement.

Answer 5: Left homonymous hemianopia may be seen with herniation of the right uncus. Uncal herniation can lead to ipsilateral compression and infarct of the PCA. This leads to contralateral homonymous hemianopia.

Bullet Summary:
Uncal herniation may present with hemiplegia (contralateral > ipsilateral), ipsilateral oculomotor nerve palsy, and ipsilateral posterior cerebral artery infarct.